:truber managed to get an ASan build working(ish).

ATM it crashes on launch.

WRITE of size 8 at 0x60200001a574 thread T185 (Gecko)
    #0 0x78cc15516e25 in memcpy /usr/local/google/buildbot/src/android/llvm-r316199/toolchain/compiler-rt/lib/asan/../sanitizer_common/sanitizer_common_interceptors.inc:739:5
    #1 0x78cc08bd23b1 in __construct_backward<unsigned int> src/android-ndk/sources/cxx-stl/llvm-libc++/include/memory:1652:17
    #2 0x78cc08bd23b1 in __swap_out_circular_buffer src/android-ndk/sources/cxx-stl/llvm-libc++/include/vector:887
    #3 0x78cc08bd23b1 in __push_back_slow_path<unsigned int> src/android-ndk/sources/cxx-stl/llvm-libc++/include/vector:1571
    #4 0x78cc08bd23b1 in push_back src/android-ndk/sources/cxx-stl/llvm-libc++/include/vector:1608
    #5 0x78cc08bd23b1 in InitDyn src/mozglue/linker/CustomElf.cpp:418
    #6 0x78cc08bd23b1 in CustomElf::Load(Mappable*, char const*, int) src/mozglue/linker/CustomElf.cpp:219
    #7 0x78cc08bd4058 in ElfLoader::Load(char const*, int, LibHandle*) src/mozglue/linker/ElfLoader.cpp:486:26
    #8 0x78cc08bde85c in __wrap_dlopen src/mozglue/linker/ElfLoader.cpp:66:51
    #9 0x78cc08bde85c in dlopenLibrary src/mozglue/android/APKOpen.cpp:189
    #10 0x78cc08bde85c in loadNSSLibs() src/mozglue/android/APKOpen.cpp:255
    #11 0x78cc08bde751 in loadSQLiteLibs src/mozglue/android/APKOpen.cpp:237:7
    #12 0x78cc08bde751 in Java_org_mozilla_gecko_mozglue_GeckoLoader_loadSQLiteLibsNative src/mozglue/android/APKOpen.cpp:301
    #13 0x78cbf6c14ef5  (/data/app/org.mozilla.fennec_aurora-2/oat/x86_64/base.odex+0x841ef5)

0x60200001a574 is located 4 bytes inside of 16-byte region [0x60200001a570,0x60200001a580)
allocated by thread T185 (Gecko) here:
    #0 0x78cc15572bd8 in operator new(unsigned long) /usr/local/google/buildbot/src/android/llvm-r316199/toolchain/compiler-rt/lib/asan/asan_new_delete.cc:92:3

Thread T185 (Gecko) created by T0 (a.fennec_aurora) here:
    #0 0x78cc1555672d in pthread_create /usr/local/google/buildbot/src/android/llvm-r316199/toolchain/compiler-rt/lib/asan/asan_interceptors.cc:204:3
    #1 0x78cc1040600a in art::Thread::CreateNativeThread(_JNIEnv*, _jobject*, unsigned long, bool) (/system/lib64/libart.so+0x4ff00a)

I think perhaps this is a false positive as described in the last paragraph here[0].

[0] https://github.com/google/sanitizers/wiki/AddressSanitizerContainerOverflow

That did it, thanks :)